The invention relates to a parting compound for the hot forming of encased metal parts and also a process for manufacturing the parting compound.
The metal parts to be formed are made in particular from reactive metals, which at elevated temperatures are susceptible to corrosion and which are to be brought into foil form. Hot forming is preferably performed by means of a conventional hot rolling apparatus. The reactive metal together with a case and a parting compound forms a sandwich-like packet. The case, which is welded together from at least two parts (cover plates), seals the reactive metal in a gas-tight manner from the environment. The parting compound, which behaves inertly in comparison with the reactive metal, ensures that a metallurgical connection does not occur between the reactive metal and the case during the hot forming. During hot rolling, the parting compound deforms plastically under the pressure produced or it liquefies like a highly-viscous fluid (i.e., having the capability to fill empty spaces under its own weight) during this a uniform, coherent parting layer is always present between the metals. On the other hand the parting compounds are brittle at ambient temperature, so that the reformed metals can be easily separated from one another.
Such a process for the manufacture of foil is known from EP-A-0374094, which is incorporated herein by reference,: A sheet-like metal piece is surrounded by a case, which is composed of two cover plates and a frame-shaped separator. The reactive metal, from which this metal piece is made, is, for example, a titanium aluminide (Ti.sub.3 Al base alloy), which can be reformed at roughly 600.degree. C.-1200.degree. C. Metal halides are proposed as the material for the parting compound; they are inert when compared with titanium base alloys. The parting compound is filled into cavities in the cover plates of the case, by which layers of 0.4 to 2 mm thick are produced. The parting compound material, which exists as a powder, is preferably applied by a thermal spraying process (e.g. atmospheric plasma spraying); in this case, air inclusions are advantageously avoided. Porous layers containing air are produced in other application processes, in which the material is spread or sprayed in the form of an aqueous solution or as a slurry.
Further functions, which are important for the reforming process, can be associated with the parting compound. The parting compound acts as thermal insulation, which advantageously keeps a heat dissipation from the metal piece to be reformed to the rolling device within certain limits. (Cf. U.S. Pat. No. 4,966,816, which is incorporated herein by reference, where a thermally insulating, fibrous intermediate layer is described, which is provided in addition to the parting compound.) The parting compound may also have the advantageous action that the reforming shearing forces act in a gentle manner on the metal piece at high hydrostatic pressure; therefore the risk of cracking is reduced. These actions occur in a particularly distinctive manner in the case of thick parting compound layers. In the known reforming process it is expensive to manufacture a thick parting compound layer because of the special formation of the case.